1. Field of the Invention
This invention relates to an optical pickup apparatus and an optical grating assembly for the apparatus. More particularly, this invention relates to an optical pickup apparatus for reading information stored in an information carrier disc by directing small read-out spots of light to the disc and detecting the light reflected therefrom by a photosensitive detector, and an optical grating assembly suitable for use in such pickup apparatus.
2. Description of the Related Art
Information carrier discs are known in the art where data are stored and arranged along a spiral track having a succession of reflective elements. The reflective elements on the track generally are in the form of hollows called "pits", which hold audio, video or other information in a digital form. The information carriers, which are also dubbed as optical discs, are finding their way rapidly into the music, movie and computer industries.
In order to read out the information stored in the carrier disc, an optical read-out or pickup device is used. FIG. 1 schematically illustrates the arrangement of a conventional optical device which employs a three-beam method for tracking servo control and an astigmatic detection for focusing servo. Before a detailed description of FIG. 1, is provided the three-beam method and astigmatic detection are briefly explained with reference to FIGS. 2 and 3.
In FIG. 2, the three-beam method for tracking servo control uses three beams of light: a main beam concentrated into a main reading spot 18a and a pair of sub-beams focused into a pair of sub-reading spots 18b and 18c on the opposite sides of the main reading spot 18a. A tracking error signal is obtained by sensing the amount of difference between the pair of reflected sub-beams.
In FIG. 3, the astigmatic detection for focusing servo control makes use of a cylindrical lens which acts as a lens with respect to light in one direction but does not act as a lens with respect to light in the opposite direction. A beam of light passing through the cylindrical lens forms a circular spot at the focal point, and distorted circular spots or elliptical spots on the far and near sides of the focal point. Any variation in shape of the reading spot of beam is electrically detected to thereby generate a focus error signal.
Now referring to FIG. 1, the optical pickup or read-out device includes a lens source. A laser beam 16 emitted by the laser source 1 is directed to a diffraction grating 12 where it is diffracted into a zero-order diffracted main beam 17a and a pair of first-order diffracted sub-beams 17b and 17c. The main beam 17a is for reading out the pit information recorded on the disc and for sensing the focusing error, while the two sub-beams 17b and 17c are for sensing the tracking error or failure of the main reading beam. The three diffracted beams 17a, 17b and 17c are directed to a planar beam splitter 13 provided with a half-silvered surface or mirror 13a where they are reflected toward a collimating lens 5. The three beams pass out of the collimating lens 5 in parallel. The collimated beams continue to an objective lens 6 which focuses the beams into three spots 18a, 18b and 18c on the surface of the information carrying disc 7 in a pattern as shown in FIG. 2.
The main reading spot 18a is derived from the main beam 17a, whereas the sub-spots 18b and 18c are delivered from the sub-beams 17b and 17c. The spot forming beams 17a, 17b and 17c are modulated by the data carrying track on the disc and reflected back through the objective lens 6 and the collimating lens 5 toward the beam splitter 13, following substantially the identical path. The beam splitter is disposed oblique or at an angle with respect to the optical path of the returning beams, while the returning beams impinge obliquely on the beam splitter 13 and pass therethrough. The length of the optical paths, for example, l and m within the beam splitter for the light beams passing therethrough vary depending on the locations at which those beams strike the splitter. The net result is similar to sending the beams through a cylindrical lens, and, thus, astigmatism is developed. The returning beams transmitted through the beam splitter 13 proceed to a plane concave lens 14 by which the incident beams are axially or longitudinally magnified and focused into spots 19a, 19b and 19c onto a six-segment photosensitive detector 15. These beam spots 19a, 19b and 19c are projected on the photodetector 15 in various patterns as shown in FIGS. 4A-4C depending on the positions of the disc 7 relative to the objective lens 6. It is noted that the spot 19a is derived from the main beam 17a, while the spots 19b and 19c are derived from the sub-beams 17b and 17c, respectively.
Turning to FIGS. 4A-4C, the photodetector 15 includes photosensitive segments A-D arranged in a square configuration to receive the main spot of beam 19a, and a pair of similar photosensitive segments E and F disposed on both sides of the square array if segments A-D for receiving the sub-spots 19b and 19c of the beam. When the surface of the information carrying disc 7 is situated in the focal plane of the objective lens 6 and it is thereby made possible for the reading spots to be focused in the data track of the disc, the beam spots 19a-19c are projected on the photodetector in the shape of round circles and in the pattern as shown in FIG. 4B. If the disc surface is closer to the objective lens beyond its focal point, i.e. if the disc surface is positioned on the near side of the focal point of the objective lens, then the projected beam spots take the form of ellipses or oval circles arranged as shown in FIG. 4A. On the other hand, when the disc surface is farther than the focal point of the objective lens i.e. when the disc surface is positioned on the far side of the focal point of the objective lens, the beam spots 19a-19c are projected on the photodetector 15 in the form of oval circles and in a pattern as shown in FIG. 4C.
A pit signal RF obtained by reading the pits in the data track of the disc with the read-out light spots is expressed by the following equation: EQU RF=a+b+c+d
where a, b, c and d represent electrical signals corresponding to the amount of light received on the photosensitive segments A, B, C and D, respectively.
A tracking error signal TES provided by the three-beam method is expressed as follows: EQU TES=e-f
where e and f represent electrical signals proportional to the amount of light projected and received on the photosensitive segments E and F, respectively.
A focus error signal FES generated by the astigmatic detection is expressed as follows: EQU FES=(a+d)-(b+c).
Whenever a tracking error is sensed by a tracking error detector 41, a lens actuator or a driver 44 operates in response to the output signal from the detector 41 to move the objective lens 6 in a direction and an amount to correct the error. Likewise, when a focusing error is sensed by a focusing error detector 42, a lens actuator or a driver 43 operates responsive to the output signal from the detector 42 to move the objective lens 6 in a direction and an amount to correct the focusing error. In this manner, the position of the objective lens 6 relative to the disc 7 is always adjusted so that the disc surface is kept in the focal plane of the objective lens and the reading beam spots are focused right on the data track of the disc, so that accurate and reliable read-out of the pits in the data track are assured.
In FIG. 5, there is schematically illustrated another conventional optical pickup device which incorporates a holographic grating. The pickup device relies on the push-pull method for tracking servo control and on the wedge prism method for focusing servo control.
Before a description of the optical pickup device of FIG. 5, is provided the push-pull and wedge prism methods will be briefly explained.
Turning to FIG. 7, the push-pull detection employs a single spot of beam for reading the information stored in the form of a pit on the disc. When the reading spot illuminated on the information track is in perfect registration with the data pit as represented by the center spot in FIG. 7, two photosensitive segments of a photodetector 45 receive equal amounts of reflected light. However, any lateral deviation of the projected light spot with respect to the data pit causes a difference in the intensity of light impinging on the photosensitive segments. This difference sensed by the photodetector 45 produces a signal indicating the tracking error.
The wedge prism method for focusing servo control employs a wedge prism 46 as shown in FIG. 8A. One face of the prism is contoured in the form of a V shaped valley. A beam of light incident on the wedge prism emanates therefrom in two beams of light as illustrated in FIG. 8B. At its focal point F, the transmitted beams are converged into tiny spots, while at positions X and Y on the near and far sides, respectively, of the focal point, the beams form semicircular spots of different size and orientation. The varying semicircular spots of the light beams are sensed by a photodetector, which produces electric outputs indicating focusing errors.
Now turning to FIG. 5, the optical pickup device includes a laser source 1. A laser beam 22 from the source of laser 1 is directed to the holographic grating 20. FIG. 6 illustrates the holographic grating 20 in an enlarged perspective view. The holographic grating 20 includes a glass or plastic plate member formed with a number of fine curved grooves or slits for diffracting the light beam passing therethrough. In order for the diffraction grating 20 to function like a wedge prism, the grating is divided into two sections 20a and 20b.
Turning back to FIG. 5, the holographic grating 20 diffracts the laser beam 22 from the source into several diffraction orders. Among them, only the zero-order diffraction beam continues to a collimating lens 6. The first-order diffraction beam wholly misses the collimating lens 6 because it has a larger diffraction angle. The zero-order diffraction beam passing through the collimating lens 6 is turned into parallel beams. The collimated beams are oriented to an objective lens 6 which focuses them into a spot of light on the surface of the information carrier disc 7. The impinging beam is reflected by the disc 7 and is returned to the holographic grating 20 along substantially the same optical path. The return beam is diffracted into several orders by the diffraction grating 20. The zero-order diffraction beam proceeds toward the laser source 1, while the first-order diffraction beam is directed to a four-segment photodetector 21. As stated hereinabove in connection with FIG. 6, the holographic grating 20 is divided into grating sections 20a and 20b along a line oriented in a tangential direction of the disc 7 (while the dividing line is conspicuous in the drawing for showing the boundary between the grating sections, but no such line exists in an actual holographic grating).
As can also be seen in FIG. 6, the sections 20a and 20b are formed with grating grooves of different designs or patterns so that the first-order diffracted beams emanating from these sections are converged onto different points. More specifically, the two first-order diffracted beams 24a and 24b emanate from the holographic grating 20 due to its composite structure. The beams 24a and 24b impinge on a four-segment photodetector 21 to form spots of light 25a and 25b. The formations of light spots 25a and 25b on the photodetector 21 enlarged and shown in FIGS. 9A-9C where the alphabetical letters A, B, C and D denote photosensitive segments including the photodetector 21. In an erroneous read-out situation where the disc 7 lies closer to the objective lens 6 beyond its focal point, i.e. the disc is located within the focal length of the objective lens 6, semicircular spots 25a and 25 are formed on the outermost photosensitive segments A and D as shown in FIG. 9A. When the disc 7 lies in the focal plane of the objective lens 6, i.e. the disc is at the focus of the objective lens, tiny spots of light are projected on the photodetector 21 as shown in FIG. 9B. If the disc 7 lies farther away from the objective lens beyond its focal point, i.e. if it is positioned outside the focal length of the objective lens, semicircular spots 25a and 25b are illuminated on the innermost photosensitive segments B and C as shown in FIG. 9C.
A pit signal RF obtained by optically reading the disc 7 is expressed as follows: EQU RF=a+b+c+d
where a, b, c and d represent electrical signals corresponding to the amount of light received by the photosensitive segments A, B, C and D, respectively.
A tracking error signal TES provided by the push-pull method is expressed as follows: EQU TES=(a+b)-(c+d).
The wedge prism method provides a focusing error signal FES expressed by the following equation: EQU FES=(a+d)-(b+c).
It should be noted that the composite holographic grating 20 acts optically like a wedge prism.
Referring again to FIG. 5, when a focusing error is sensed by a focusing error detector 42, a lens actuator 43 operates to drive the objective lens 6 in a direction and an amount to correct the error. Likewise, as a tracking error detector 41 senses a tracking error, another actuator 44 operates to move the objective lens 6 in a direction and an amount to offset the focusing error. Thereby, an accurate and a reliable reading of the pits on the disc is achieved.
The conventional optical pickup devices of the type described above, while generally satisfactory in optical read-out operation, suffer some drawbacks.
The optical pickup device of FIG. 1 which employs the three-beam detection for the tracking servo control provides an excellent ability to detect tracking error. However, this device requires both the diffraction grating 12 and the plane beam splitter 13 for the diffraction of the laser beam, as compared with a single holographic grating for the optical pickup device of FIG. 5 based on the push-pull detection method. One additional component part means one additional assignment of operational adjustment as well as additional costs.
On the other hand, in the pickup device of FIG. 5 which relies on the push-pull detection technique, a single holographic grating 20 is sufficient for the intended diffraction of the laser beam. One fewer component part and one fewer assignment for the operational adjustment in this pickup device is used than in the device based on the three-beam detection which leads to a considerable cost reduction. However, the push-pull type of the pickup device is disadvantageous in that the tracking error signal fluctuates with varying pit depths and it is impossible to obtain a stable and constant tracking error signal.